Neutron Dosimetry with a Planar Silicon pin Diodes'

1
Neutron Dosimetry with a Planar Silicon p-i-n
Diodes.
A.B. Rosenfeld1, M Yudelev2, M.L.F. Lerch1, I
Cornelius1, V.L.Perevertailo3, I.E. Anokhin4,
O.S. Zinets4, V.I.Khivrich4, M.Pinkovskaya4, P
Griffin5, D Alexiev6, M Reinhard6
1. Centre for Medical Radiation Physics,
University of Wollongong 2. Gershenson Cancer
Research Centre , Wayne State University ,
Detroit, USA 3. SPA Detector/BIT, Ukraine 4.
Institute for Nuclear Research, Ukraine 5. Sandia
National Laboratoty, USA 6. Australian Nuclear
Science and Technology Organization, Australia
2
Aims and Applications of Research

• Revisit bulk p-i-n application as a neutron
  (NIEL) dosimeter
• Development of a simple sensor with wide dynamic
  range that is required for measurement of NIEL
  and IEL
• Investigate the optimal design of new
  multi-range, planar p-i-n diode for simultaneous
  IEL and NIEL measurements.
• Study the possibility of simultaneous
  measurements of NIEL and IEL with the same
  sensor.
• Diodes to be applied for the practical and simple
  measurement of non ionizing energy losses (NIEL)
  and ionizing energy losses (IEL) in neutron,
  gamma and proton fields. This is an important
  issue for quality assurance in a space
  environment and radiation environments at
  different radiation facilities.

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Method of NIEL measurements

• NIEL the operation of the low resistivity pin
  diode with long base sensor is based on the
  change of forward voltage
• the disadvantage of this sensor is in the low
  dynamic range due to saturation of t

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Method of IEL measurements

• Photodiodes disadvantage in radiation damage in
  changing of sensitivity due to NIEL radiation
  damage
• MOSFET sensor are used for integral IEL dosimetry
  and has limited range for on-line dose rate
  measurements of IEL

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Solution

• Use high resistivity pin diodes with a long base
  to compensate for the saturation of t with
  increasing resistivity of the base during the
  irradiation.
• A theoretical simulation is required to find the
  optimal design of such a diode with a wide dose
  range.
• Simplify main features of long base, bulk p-i-n
  diodes by fabrication of planar p-i-n diode for
  NIEL and IEL
• Simulate the optimal geometry of diodes for wide
  range of applications

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Bulk and Planar Diode designs
D-Type
p
n-silicon
L-Type
n
C-Type
G-Type
a refers to the radial base length
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The theoretical p-i-n sensor sensitivity for
different initial Si. The sample parameters
I  1 mA, L  1 mm, w  1mm, d  300 ?.
Theoretical modelling of planar structures
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Radiation Facilities

• The new diodes were irradiated
• (1) in a neutron field at SNL Pulse Reactor
  Facility (SPR-III)
• (2) fast neutron therapy facility (Detroit) with
  the maximum energy of the neutrons of 48.5 MeV
  and
• (3) with 3 MeV protons at ANSTO, (Australia)

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Schematic diagram showing the proposed new single
sensor system for the measurement of NIEL and IEL
in silicon with adjustable sensitivity.
Application of new planar diodes
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On-line Dosimetry System
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Response a p-i-n diode in the neutron field of
SPR-III

• On-line neutron dose measurements
• D-type diodes
• length of base 1.2 mm

r 1500 Wcm
r 40 Wcm
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Response a p-i-n diode in the neutron field of
SPR-III

• On-line measurements
• Neutron dose rate 0.15 cGy/s (Tissue)
• Period of measurements 20s

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D-type diode response at FNT Facility
D-type diode in 15cm x15cm open field and depth
5cm in water. The total neutron dose delivered
was 50 cGy. Gamma dose 6
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D-type diode response at FNT Facility
Measurements taken using D-type diode in 30cm
x30cm blocked field and depth 5cm in water. The
total neutron dose delivered was 16.6 cGy. Gamma
dose 30
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Dose rate dependence
Sensitivity of the diode versus accelerator
current for dose increment of 25 MU 21cGy at
the point of irradiation.
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Response of C-type diode in build up region
Comparison of the relative total depth dose
distribution in A-150 phantom obtained with TE
ionisation chamber and response of C2 p-i-n diode.
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On-line neutron beam profile measurement
On-line neutron beam profile measurement of a
10x10 cm2 neutron field with detectors at a depth
of 5 cm in a water phantom
D-Type bulk diode
C-Type planar diode in edge-on mode
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Neutron response of C-Type p-i-n diodes
Neutron response of C-1 at depth 5 cm in a water
for two readout currents 1 and 20 mA. The
sensitivity is 0.14 mV/MU and 0.30mV/MU at point
of irradiation.
Neutron response of C-2 at depth 5 cm in a water
for two readout currents 1 and 20 mA. The
sensitivity is 0.88 and 3.32 mV/MU for C-2 diode.
1 MU 1cGy at point of irradiation
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Neutron response of L-type p-i-n linear diode
array

• Dose increments were 15 MU
• Readout current was 0.16mA for first and second
  and third p pad.
• All measurements done at depth of 1 cm in A-150
  plastic phantom

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Neutron and proton sensitivity of linear array
diode
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IEL response of neutron irradiated diodes

• Optical image of C2 device, showing central p
  region and outer n ring.
• IBIC scan area is the superimposed white square.

• IBIC image of median energy event at each pixel
  of scan for device C2 at reverse bias of 0V and
  400V (After diode was irradiated with 3 x1011
  n/cm2)

Energy event spectra for IBIC scan of C2 device
at reverse bias of 0V and 400V
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Dual mode of operation of pin diode

• Voltage drop mode response

• Current (charge) mode response

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Measurement of diode sensitivity in a charge mode

• Response of TE chamber in mixed gamma neutron
  field

• Response of the pin diode in a charge mode in
  mixed
• gamma neutron field

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Lead attenuation method

• Method of measurements of Cn,mv and Cn,nC
• Average neutron energy is 20 MeV

40 cm
182.9cm
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Schematic diagram of readout circuitry
Simultaneous measurements of cyclotron beam
target current and voltage drop on a pin diode
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Lead attenuation method

• The transmission of the d(48.5)-Be neutron beam
  through lead measured with the TE ionization
  chamber (?) and the diode operated in charge mode
  (?).

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Neutron sensitivity Cn,mv of C-1 and C-2

• C1
• 0.109 mV /cGy
• C2
• 1.27 mV/cGy
• Ratio of sensitivities is proportional to t2

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Neutron sensitivity of the diode vs readout
current

• Sensitivity
• 1mA
• 0.109 mV/cGy
• 8 mA
• 0.586 mV/cGy

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Separate gamma and neutron dosimetry
From the voltage drop mode
From the charge mode
Total dose
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Measurement of the neutron dose profileC1diode

• Depth 5cm
• Field10x10cm2

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Total dose profile based on dual mode

• Comparison of total dose profile measurements
  using gamma and neutron dose from a single diode
  C1..
• Depth5cm
• Field10x10cm2

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Neutron and total depth dose profile

• Neutron dose profile for 10x10 cm2 field.
• Total dose profile
• for 10x10 cm2 field

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Separate neutron and gamma dosimetry

• Diode C2
• Open field
• 10x10 cm2

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Separate neutron and gamma dosimetry

• Diode C1
• Field 30x30 cm2
• Partially blocked with
• 93.5mm tungsten.
• Increasing of gamma
• component for 16.

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Separate neutron and gamma dosimetry

• Diode C2
• Open field
• 10x10 cm 2

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Separate neutron and gamma dosimetry

• Diode C2
• Field 30x30 cm2
• Partially blocked with
• 93.5mm tungsten.
• Increasing of gamma
• component for 16.

37
New RDM MOSFETS with flexi carrier
Opening for Al pads in place of Si chip contact
pads
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Conclusion

• We have demonstrated that it is possible to
  replace traditional bulk p-i-n diodes with new
  planar diodes for application in NIEL and IEL
  measurements
• C-type diodes have a predictable response, a2 ,
  while p-i-n diode array do not appear to follow
  a2 law.
• Both the bulk and planar diodes are capable of
  on-line application
• The sensitivity of planar diodes can be easily
  adjusted two orders of magnitude
• The planar diodes are capable to measure
  simultaneously NIEL and IEL
• The C-type diodes have shown to be the preferable
  design

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Conclusion

• In measurement of gamma dose diode was accurate
  to within 2.5cGy. However TE/GM pair detector
  have an accuracy about 8.
• New packaging for RDM sensors suitable for HEP
  has been developed make RDM sensor as a standard
  device for HEP and radiation hardness testing
  applications.